A d-c terminal voltage of a converter bridge circuit, which is connected to the a-c voltage side to the individual phase conductors of a three-phase network, is determined by the phase of the respective phase conductor voltage at which a bridge arm connected to this phase conductor is fired and conducts current. By presetting a control angle or a control voltage for firing the converter valves with reference to the three-phase network, it is thereby possible to preset a desired d-c terminal voltage to the connected d-c circuit or to control the current flowing through the converter by a superimposed current control. In a symmetrical voltage system of the three-phase network, equal time intervals for the firing instants of the bridge arms are then obtained in the steady-state condition. The bridge arms are therefore fired with equidistant firing angles, with reference to a voltage synchronous with the network.
However, the symmetry of the network is disturbed if there is a short circuit in the three-phase network. If the converter valves receive a common control angle for all bridge arms, a smaller d-c terminal voltage occurs in the d-c circuit on which an undesired pulsating component is superimposed. For an inverter, a problem occurs in that the current flowing through the one valve of a bridge arm pair increases like a short circuit and is not commutated from this valve fast enough to another bridge arm pair before the other valve of the first bridge arm pair is fired. A so-called inverter flipping then occurs where both valves of a bridge arm pair carry current and short the d-c terminal voltage.
The converter must therefore be blocked until the network fault is eliminated in the event of network shorts or other asymmetries of the three-phase network. In this way, the power transmission via the converter is interrupted for an extended period of time. However, it is frequently desirable to operate a machine connected to the converter at least with reduced power until the undisturbed network voltage recurs. One particular field of application is the high-voltage d-c transmission (HVDC) between two three-phase networks which are connected to each other by a first converter operated as a rectifier, a transmission line and a second converter operated as an inverter. For HVDC transmission, it is desirable that in the event of network disturbances which are present for several periods of the three-phase networks, the power transmission is not completely interrupted so that the undisturbed network is influenced as little as possible by the disturbance in the other network. A minimal disturbance of the basically undisturbed network can also be sought if this network is connected via the converter to another short circuit-prone load such as a machine.
A problem, therefore, exists in the regulation or control of energy transmission through a converter when the voltage system of a three-phase network coupled to the converter is asymmetrical.